1. Field of the Invention
The invention is related to the field of pulsed neutron well logging. More specifically, the invention is an apparatus and method for improving accuracy and precision of capture cross-section measurements.
2. Description of the Related Art
Pulsed neutron well logging instruments are known in the art for determining the macroscopic thermal neutron capture cross-section of earth formations penetrated by a wellbore. Typical pulsed neutron instruments include a controllable source of high-energy neutrons and one or more gamma ray radiation detectors positioned at spaced apart locations from the neutron source. The source is periodically activated to emit controlled-duration “bursts” of high-energy neutrons into the earth formations surrounding the well borehole. These neutrons interact with the atomic nuclei of the materials in the earth formations, losing energy with each interaction until the neutrons reach the thermal energy level (defined as having a most likely energy of about 0.025 electron volts). Depending on the material composition of the earth formations proximal to the instrument, the thermal neutrons can be absorbed (captured), at various rates by certain types of atomic nuclei in the earth formations. When one of these atomic nuclei captures a thermal neutron, it emits a gamma ray, which is referred to as a “capture gamma ray”.
The rate at which the capture gamma rays are emitted, with respect to the elapsed time after the end of the neutron “burst” depends on, among other things, the relative concentration per unit volume in the earth formations of those atomic nuclei which have a relatively large tendency to absorb thermal neutrons and emit capture gamma rays in response. This tendency is referred to as the thermal neutron capture cross-section.
The capture cross section, designated as Σ, of the formation is determined by sending a burst of neutrons from the tool and watching the decline of the gamma ray count rate with time as the neutrons are captured by the surrounding materials (neutron capture) and as they diffuse farther away (neutron diffusion). Σ is inferred from this observed decline in the gamma ray count rate versus time. However, in addition to the neutron capture, two key environmental effects contribute to the observed decline, or decay rate: diffusion and the so-called “borehole contamination.” These effects need to be carefully characterized in order to determine the correct Σ throughout the wide range of operating conditions typically encountered in the oilfield. These effects are controlled by parameters which include borehole size, casing size, casing weight, borehole fluid salinity, porosity, and lithology.
Some approaches to handling these environmental effects have been described in the prior art literature. See, for e.g., , Steinman, et al. “Dual-Burst Thermal Decay Time Logging Principles,” 1986 SPE Annual Technical conference and Exhibition, New Orleans, La., (Oct. 5-8, 1986), paper SPE 15437; Smith, et al., “Obtaining Intrinsic Formation Capture Cross Sections with Pulsed Neutron Capture Logging Tools,” Transactions of the 29th Annual SPWLA Symposium, San Antonio, Tex. (Jun. 5-8, 1988), paper SS; Murdoch, et al., “Diffusion Corrections to Pulsed Neutron Capture Logs: Methodology,” Transactions of the 31st Annual SPWLA Symposium, Lafayette, La. (Jun. 24-27, 1990), paper Q; and Odom et al., “Quantitative Use of Computer Models in Calibration of the Computalog Pulsed Neutron Thermal Decay, Transactions of the 33rd Annual SPWLA Logging Symposium, Oklahoma City, Okla. (Jun. 14-17, 1992), paper P.
Under certain wellbore conditions, it is difficult to determine the fractional saturation of oil or gas by processing the capture gamma ray measurements according to methods known in the art for determining the thermal neutron capture cross-section, ΣF, of the earth formation of interest. Several factors contribute to this difficulty. First, the well logging instrument is typically inserted into a wellbore which is filled with liquid. At the time the pulsed neutron instrument is typically used, the wellbore generally has inserted therein a steel liner or casing. The liner or casing is generally held in place by cement filling the annular space between the wellbore wall and the exterior of the liner or casing. As high-energy neutrons leave the neutron source in the logging instrument, the mud in the wellbore has the effect of rapidly moderating (or slowing down) the high-energy neutrons to the thermal level due to of the high concentration of hydrogen nuclei in the mud.
In general, the relative numbers of thermal neutrons (“population”) at any particular time after a neutron burst, or thermal neutrons in the wellbore and in the earth formations proximal to the wellbore, will depend on the porosity and on the hydrogen nucleus concentration within the earth formation. This population can be “captured,” or absorbed by nuclei of various chemical elements in the wellbore and formations, at a rate which depends upon the relative concentration and on the thermal neutron capture cross-section of these elements. In wellbores and in earth formations, some of the more common elements having high thermal neutron cross-sections include chlorine, hydrogen, iron, silicon, calcium, boron, and sulfur. As determined from measurements of capture gamma rays made by the well logging instrument, the thermal neutron decay time (neutron lifetime), represents combined effects of the thermal neutron capture cross-section in each of several regions within the wellbore as well as in the earth formations proximal to the wellbore. These regions include the instrument itself, the fluid in the wellbore, the steel casing, the cement, the earth formations radially proximal to the wellbore wall (which may have been infiltrated by fluid from within the wellbore), and the earth formations radially more distal from the wellbore wall (which have minimal infiltration from the fluid in the wellbore).
Several prior art are aimed at improving measurements in capture cross-section logging. A method and apparatus employing a source and two detectors are discussed, for example, in U.S. Pat. Nos. 4,645,926 and 4,656,354, both issued to Randall. A subsurface instrument includes a long-spaced (LS) and short-spaced (SS) detector for detecting natural or induced gamma ray emissions from subsurface formations. The detectors produce electrical pulses, with each pulse corresponding in time with the incidence of a corresponding gamma ray on the detector and having an analog voltage amplitude correlative of the gamma ray. A method is discussed in Randall '354 for determining presence of a gas by comparing first and second parameters obtained at the detectors. The first parameter is indicative of a count of detected impingements of primarily inelastic gamma radiation upon a detector. The second parameter is indicative of a count of detected impingements of primarily capture gamma radiation upon a detector. Randall '926 discusses a method of determining a parameter of the borehole, wherein primarily inelastic gamma radiation is normalized upon impingement on a detection means.
In U.S. Pat. No. 4,668,863, issued to Gray, et al., an apparatus is used to analyze and process parameters including, for example, the macroscopic thermal neutron absorption capture cross-section of the formation at borehole elevations corresponding to the locations from which such spectra are derived. For acquiring temporal spectral data, a multi-channel scale section is provided which includes a channel number generator which produces a numerical sequence of memory address codes corresponding to a sequence of adjacent time windows. A suitable memory device is part of the downhole apparatus. Each code uniquely defines a start time, whereby the windows collectively comprise the time interval of the desired spectrum. Each time a gamma ray pulse is detected, the memory address generated at that time addresses a corresponding memory location and increments the count value resident therein. At the conclusion of the time spectrum interval of interest, the memory locations may be interrogated by the CPU and the resultant spectral data analyzed, transmitted to the surface, or presented visually as a gamma ray emission count versus time plot. Correlation is made of detection signals in response to impingement of gamma radiation upon first and second detectors.
U.S. Pat. No. 5,973,321, issued to Schmidt, discusses a method for determining the fractional amounts and the thermal neutron capture cross-sections of various regions in a wellbore and regions in earth formations in the vicinity of the wellbore, each having a distinct mean neutron decay time or macroscopic thermal neutron capture cross-section. The method includes generating a data kernel which is made up of representors (models), or potential decay components of the wellbore and of the earth formations in the vicinity of the wellbore. A thermal neutron decay spectrum is measured by a pulsed neutron instrument including a controllable source of high-energy neutrons and one or more gamma ray detectors at spaced-apart locations from the source. The decay spectrum measured by the instrument is inverted to determine model parameters by which the individual representors are scaled so that when combined, the scaled representors most closely match the measured decay spectrum.
U.S. Pat. No. 5,808,298, issued to Mickael, discusses a method for determining the oil saturation in an earth formation penetrated by a wellbore. Measurement of the relative amounts of carbon and oxygen in the wellbore and formation are made by spectral analysis of neutron-induced inelastic gamma rays detected from the earth formation at spaced-apart locations. The method includes calculating an apparent oil holdup in the wellbore at each one of the spaced-apart locations from the measurements of the relative amounts of carbon and oxygen. A corrected oil holdup is calculated in the wellbore from differences between the apparent holdups determined at each of the spaced-apart locations. An apparent oil saturation in the formation is determined at each of the spaced-apart locations from the relative amounts of carbon and oxygen and the corrected oil holdup. A corrected formation oil saturation is determined from differences between the apparent oil saturations at each of the spaced apart locations.
Plasek et al., 1995, “Improved Pulsed Neutron Capture Logging With Slim Carbon-Oxygen Tools: Methodology,” SPE Annual Technical Conference & Exhibition, Dallas, Tex., Oct. 22-25, 1995, paper SPE 30598, discusses a method of carbon-oxygen ratio determination using a reservoir saturation tool for determining physical parameters of a formation. In one embodiment of the tool, the source and detectors are cylindrically symmetrical. In an alternate embodiment of the tool, the short-spaced detector is positioned so as to face toward the borehole while the long-spaced detector is positioned against a face of the borehole wall so as to receive signals from inside a formation layer.
A need exists for improving a precision and accuracy of parameter estimation obtained from earth formations. The present invention fulfills this need.